Draft: Add FF to toggle /code_suggestions/tokens endpoint

What does this MR do and why?

• There is already a feature flag called code_suggestions_tokens_api that lets us toggle this endpoint and the code completions endpoint, but this allows us to turn off just the /code_suggestions/tokens endpoint
• We may want to turn this off before removing it entirely because it is an endpoint for a legacy codepath that we no longer want to support.
• The tokens generated by this endpoint will soon be un-usable because the AI Gateway will be updated to only accept requests from workhorse: gitlab-com/gl-infra/platform/runway/runwayctl!159 (diffs)
• By turning this endpoint off with a feature flag, we make sure that there aren't any unexpected consequences when turning off the endpoint (such as raising errors from plugins)

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